As mounting space is limited in a small sized wireless mobile terminal, interference caused by electromagnetic coupling or capacitive coupling among an antenna and each portion of a circuit of the wireless mobile terminal may cause problems. In particular, the antenna may suffer from a reduction of radiation efficiency. For those problems, possible solutions of related art using magnetic material have been proposed as described hereafter.
A first possible solution is disclosed in Japanese Patent Publication of Unexamined Application (Kokai), No. 2001-156484, as to a mobile communication apparatus including a printed circuit board, a shield case for shielding a portion of the printed circuit board, and an antenna which may be pulled out of the shield case to be extended.
According to the above first solution, a shield effect may be improved by using two methods. One of the two methods is to strengthen electrical connections between the shield case and a ground pattern of the printed circuit board in a direction perpendicular to a direction of a radio frequency current induced on the shield case. Another one of the two methods is to layer magnetic films having an axis of easy magnetization in the direction of the radio frequency current induced on the shield case.
A second possible solution is disclosed in Japanese Patent Publication of Unexamined Application (Kokai), No. 2003-198412, as to a mobile communication apparatus including anisotropic magnetic material in a near magnetic field produced by the apparatus.
According to the above second solution, the anisotropy may be directed in a same direction as magnetic field lines forming the radio frequency magnetic field are, so that the magnetic field may be absorbed by the anisotropic magnetic material.
A third possible solution is disclosed in Japanese Patent Publication (Toroku), No. 3713476, as to a mobile communication apparatus including a built-in L-shaped antenna, a printed circuit board facing the antenna, and a plate of magnetic material that is laid on the printed circuit board.
The mobile communication apparatus of the above third solution may reduce magnetic field strength on a surface of a ground layer of the printed circuit board, may reduce induced currents and may make antenna directivity stable.
A wireless mobile terminal may include a built-in antenna of a complex shape like being folded or branched so as to meet one need for a smaller size and a thinner shape of the wireless mobile terminal and another need for multi-resonance and a broader frequency range, which tend to conflict to each other. Above wording of “built-in antenna” means an antenna provided inside a housing of the wireless mobile terminal, or an antenna unitarily formed as a portion of an inner or outer face of the housing.
The built-in antenna may suffer from a reduction of radiation efficiency due to the above complex shapes. Upon including an element folded 180 degrees, e.g., the built-in antenna may suffer from a reduction of radiation efficiency, as antenna currents distributed on both sides of a fold portion are spatially directed in reverse to each other.
Upon being of a meander type which is well known for space efficiency, the built-in antenna may suffer from a reduction of radiation efficiency, as antenna currents distributed on portions neighboring to each other are spatially directed in reverse to each other. Upon including an element that branches into two parallel portions, the built-in antenna may suffer from an impedance mismatch due to capacitive coupling between the two parallel portions.
The first solution of the related art described above is of a wireless mobile terminal having an extendable antenna. This wireless mobile terminal may be configured to have lower impedance of the shield case so that a radio frequency current may easily flow on the shield case, and that the radio frequency current may keep from being conducted into the portion shielded by the shield case.
The first solution of the related art may hardly be applied to a wireless mobile terminal including a built-in antenna, as the antenna and the printed circuit board are relatively positioned in a manner different from those of the wireless mobile terminal having the extendable antenna. The first solution of the related art may not be applied in a case where it is difficult to define the direction of the axis of easy magnetization uniquely, as the direction of the axis of easy magnetization should be defined while the magnetic films are being layered.
The second solution of the related art described above is to absorb the near magnetic field of the mobile communication apparatus (a wireless mobile terminal). In order to improve radiation efficiency of a built-in antenna of the wireless mobile terminal, it is not enough only to absorb the near magnetic field. In addition, it is necessary to emit an electromagnetic field so efficiently that the built-in antenna features a required radiation pattern and a required antenna gain. That is, the second solution of the related art alone is not enough to improve the radiation efficiency of the built-in antenna.
The third solution of the related art described above is to lay the plate of magnetic material between the built-in antenna and a ground layer of the printed circuit board so as to reduce influence of an unbalanced current induced on the ground layer. The built-in antenna, however, may not be of a simple L-shape but may be of a complex shape as described earlier. That is, although possibly contributing to a thin shape of the wireless mobile terminal, the third solution of the related art may not contribute to alleviating limited mounting space for the built-in antenna or to downsizing of a mounting area for the built-in antenna.